Field of the Invention
The invention concerns a honeycomb body, in particular for an exhaust gas catalytic converter, through which a fluid can flow and which has an intake region and an outlet region. The intake region and the outlet region are connected together by passages through which the fluid can flow.
Published, International Patent Application WO 90/03220 discloses honeycomb bodies through which a fluid can flow and which can be of the most widely varying cross-sectional shapes. To reduce the polluting components in the exhaust gas flow of an exhaust gas system it is necessary for the honeycomb body to be at a temperature which is sufficiently high for the catalytic reaction of the pollutants. In order to endow the catalytic converter with a performance such that it starts operating as quickly as possible, it has already been proposed in Published, International Patent Application Wo 89/10471 that the honeycomb body is to be electrically heated. Electrical heating of such a honeycomb body reduces the emission of exhaust gas pollutants that are produced in the cold-start phase of an internal combustion engine. The internal combustion engine and the entire exhaust gas system cools down when a motor vehicle is operated in a short-distance mode with interruptions of greater or lesser lengths between the individual travel cycles. Cooling of the exhaust gas system and in particular the honeycomb body in that way has the result that the honeycomb body has to be repeatedly electrically heated. Under some circumstances the consequence of this is that the power source, a battery in a motor vehicle, is severely loaded. When the vehicle is used to cover short distances therefore it is not possible to guarantee that the battery is always sufficiently charged up again.
In order to reduce the amount of power necessary to heat the honeycomb body it has therefore already been proposed that the honeycomb body should be provided with thermal insulation. Thus for example Published International Patent Application WO 90/04087 discloses a honeycomb body with a thermal insulation which is in the form of a double casing system. That double casing system includes an inner tubular casing which is disposed in an outer tubular casing at a spacing relative thereto. The intermediate space between the inner and the outer tubular casings acts as a thermal insulation. That affords a reduction in the emission of heat by the peripheral surface of the honeycomb body.
For the purposes of reducing thermal conductivity U.S. Pat. No. 5,549,873 already discloses a honeycomb body having an intermediate region with a considerably reduced level of thermal conduction. That intermediate region is produced by openings or stamped-out portions provided in the metal sheets of the honeycomb body. The region upstream of the openings serves in practice as a pre-catalyst, that is to say that partial region of the honeycomb body forms a mass which can rapidly heat up immediately after a cold start of the engine, while in the region of the main catalyst the carrier body forms a substantially larger number of flow paths so that there too a shorter active catalytic surface area is made available.